Sewage sludge is the thick, malodorous slurry left behind in a sewage treatment plant after its load of human and industrial chemical wastes has been bio-chemically treated and the wastewater discharged.
The large amount of human waste in sewage treatment plants means that the sludge contains high concentrations of phosphates and nitrates, desirable components of fertilizer. However, the industrial wastes that are present in sewage cause toxic materials such as industrial solvents, heavy metals, and even nuclear waste to be left behind in sludge. When sewage sludge is applied to the fields, both the nutrients and the toxic chemicals are released to the environment. There are many of these toxic chemicals, and they are often found in high concentrations.
Sewage sludge solids comprise a mixture of organic materials composed mainly of crude proteins, lipids and carbohydrates, and inorganic materials, comprising significant quantities of silt, grit, clay and lower levels of heavy metals. In addition, the bacteria still alive are pathogenic and may contaminate soils and subsequently ground water. Typical raw sewage sludge comprises about 60-80% volatile material, and contains about 25-40% organic carbon (Table 1). Disposal of the sludge is expensive and normally constitutes up to 50% of the total annual costs of wastewater treatment. The major sludge disposal options currently used include agricultural utilization, land-filling and incineration (See FIG. 1).
TABLE 1Activated Sludge CharacteristicsRange ofCharacteristicvaluesTypical valueCommentspH6.5-8.0—Can be less in purity oxygen—5.5systems or if anaerobicdecomposition begins. Baltimore,MarylandHeating value, Btu/lb,—6,540 (15,200)Increases with percent volatile(KJ/kg)content.Specific gravity of individual— 1.08solid particlesBulk specific gravity—1.0 ÷ 7.0 × 10−8 × CC is suspended solids concentration,in mg/l.Color—BrownSome grayish sludge been noted.Activated sludge becomes blackupon anaerobic decomposition.Carbon/Nitrogen ratio—12.9 Baltimore, Maryland—6.6Jasper, Indiana—14.6 Richmond, Indiana—5.7Southwest plant, Chicago, Illinois—3.5Milwaukee, Wisconsin (heat dried)Organic carbon, percent by17-41—Zurich, Switzerlandweight of dry solids23-44—Four plantsNitrogen, percent by weight4.7-6.7—Zurich, Switzerlandof dry solids (expressed as—5.6Chicago, IllinoisN)2.4-5.0—Four plants—6.0Milwaukee, WisconsinPhosphorus, percent by3.0-3.7—Zurich, Switzerlandweight of dry solids as P2O5—7.0Chicago, Illinois(divide by 2.29 to obtain 2.8-11.0—Four plantsphosphorus as P)—4.0Milwaukee, WisconsinPotassium, percent by0.5-0.7—Zurich, Switzerlandweight of dry solids as K2O— 0.56Chicago, Illinois(divide by 1.2 to obtain— 0.41Milwaukee, Wisconsinpotassium as K)Volatile solids, percent by61-75—Zurich, Switzerlandweight of dry solids (percent—63.0 ash is 100 minus percent62-75—volatile)59-70—Four plants—76.0 Renton, Washington (Seattle Metro),1976 average88.0 San Ramon, California (ValleyCommunity Services District), 1975averageVolatile solids (continued)—81.0 Central plant, Sacramento County,California, July 1977-June 1978averageGrease and fat, percent by 5.0-12.0—Ether extractweight of dry solidsCellulose, percent by weight—7.0Includes ligninof dry solidsProtein, percent by weight of32-41——dry solids
Wastewater treatment plants, therefore, currently are designed to minimize sludge production and all efforts are taken to stabilize and reduce its volume prior to disposal or utilization. Furthermore, increasing sludge disposal costs and diminishing landfill capacities are continually driving interest in sludge drying. As drying reduces the bulk and weight of sludge, transport and disposal costs are significantly alleviated. Drying process is highly energy consuming and hence very expensive.
Numerous sludge processing options have been proposed and have the potential to convert a fraction of organic material into usable energy, but only a few have been demonstrated to be viable net energy produced at full scale. Anaerobic digestion of sewage sludge is probably the most common process employed to date, about 25% of the available organic materials being converted to produce a gas rich in methane, resulting in an energy production of about 5 MJ/kg of dry sludge solids fed to the digester. Other alternatives, such as air incineration, gasification and liquefaction have recently been reported as viable technologies for net energy production from sewage sludge. Mostly to process woody and lignite agricultural wastes. There is very scant information on high nitrogen containing sludges.
The idea to use the processes of synthetic oil production from solid fuels for the treatment of various organic wastes, sewage sludge included, is based on the similarity of the chemical composition of the organic matter of these fuels and that of the waste products. Table 2 shows the elemental composition of various hydrocarbon sources and other substances present in diverse organic waste, including sewage sludge (lipids, proteins, hydrocarbons).
TABLE 2Variation in Elemental Composition of Different Types of Organic MatterType ofElemental composition, % wt daforganicH/CmatterCHNSOatom.Methane87.512.5 ———1.7Crude oil84.0-87.011.0-14.00.1-0.30.5-3.51.0-3.01.5-1.9Coal66.0-86.05.7-7.00.5-1.90.4-3.5 8.0-29.00.9-1.3Oil shale62.0-80.0 7.5-10.00.5-2.51.0-1.4 6.0-15.01.1-1.4(kerogen)Peat49.0-60.05.0-8.01.0-4.00.1-1.028.0-48.00.9-1.9Wood48.0-52.05.8-6.20.1-1.5—40.0-45.01.4-1.5Cellulose44.46.2——49.41.7Lignin63.06.0——31.01.1Fats76.0-79.011.0-13.0——10.0-12.01.7-2.0Albumin's50.0-55.06.5-7.515.0-18.00.3-2.521.5-23.51.7-1.8Sewage23.0-44.04.5-6.02.5-7.50.3-1.816.0-24.01.2-1.7sludge
As can be seen from Table 2, all organic substances listed in it are composed of the same five elements in different concentrations. They differ in the structure and mass of their molecules. FIG. 2 shows the molecular models of the structure of an organic matter of bituminous coal, lignin (a natural polymer, which is a component of the wood structure) and organic components of the sewage sludge structure. It can be seen that the molecules under discussion are basically similar in structure.
They consist of ring-shaped aromatic and hydro-aromatic nuclei both single and condensed, linked by aliphatic or hetero-atomic cross links. Since such links have low energy of formation, they are the first to be destroyed by thermal treatment, and radical fragments are formed. The more said cross links there are in the structure of the material and the lower the energy of such links, the lower the temperature of their destruction point and the smaller the fragments they break into. The newly-formed fragments are chemically active radicals which in the absence of hydrogen combine (recombine) into heavy products and coke. With hydrogen from any source present, oil molecules are formed. FIG. 3 illustrates the above, which holds true for any solid fuel, including the organic matter of sewage sludge.
The process of obtaining synthetic oil from any solid organic feed consists of two basic stages:                Thermal cleavage of the macromolecular structure, with radical fragments of different size formed;        Stabilization of said radicals either through their recombination or through redistribution of hydrogen and alkyl groups in the feed stock structure, or through external introduction of hydrogen (molecular or donor).        
It follows from the above that all thermo-chemical processes of obtaining synthetic oil essentially differ only in the methods by which the formed radical fragments are stabilized:                By pyrolysis—through redistribution of the hydrogen in the organic matter of the feed stock;        By hydro-pyrolysis and hydrogenation—through external introduction of molecular hydrogen;        By thermal extraction—at the expense of hydrogen donor from the recirculating solvent.        
The advantage of the thermal extraction process as compared to the other processes mentioned above is that the recirculating solvent contains components which easily loose hydrogen (H-donors) at temperatures of the process. This donor hydrogen splits off the active atomic form and quickly and easily reacts with the radical fragments, stabilizing them in the form of liquid products.
FIG. 4 illustrates the importance of the presence of substances which act as hydrogen donors in the recirculating solvent and the principle of their interaction with the feed stock during the liquefaction process.
The naphthalenetetralinedecaline transformation activates the hydrogen at much lower temperatures (e.g. 200° C.).
One of the major problems in direct solid fuels liquefaction processes is the separation of solids, mineral matter and unconverted organic matter from liquefaction products. Solid/liquid separation processes must be used to separate the mineral residue and unconverted carbon from liquid products. Difficulties in removing these solid components represent a major obstacle to economic production of liquefied feed stocks products.
Filtration, centrifugation, sedimentation, hydro-cloning and screening are all methods for mechanical separation of solid particulates from slurries. Thereto, other methods have been sought to solve the problem: vacuum distillation and extraction methods were used to separate liquid products from solid residue. The removal of micron-sized particles is difficult to achieve. One way of proceeding is as follows: the viscosity is decreased by blending with a relatively high amount (about 40-60% wt) of low viscosity liquid solvent so that the separation of the solids by centrifugation or filtration becomes possible. At a subsequent stage of the process, the solvent is recovered by distillation. However, centrifuges wear out quickly when used for the separation of the micron-sized particles. Filters are rapidly clogged by the fine material and have to be changed frequently, thereby making the process tedious and costly.
The main shortcoming of the separation methods mentioned above is that none of them ensures complete separation of the liquid products from the solid residue: 25-40% wt of the oil obtained in the liquefaction process remains absorbed in the pores of the solid residue.
Proceeding from what is described hereinbefore, the processes used for liquefying solid fossil fuels may be used to obtain liquid synthetic products from organic waste, sewage sludge included.
An example of thermal gasification system has been proposed by S. A. Virgil and G. Tchobanoglous in a paper entitled, “Thermal Gasification of Densified Sewage Sludge and Solid Waste”, presented at the 53rd Annual Water Pollution Control Federation (WPCF) Conference at Las Vegas, Nev. USA in October 1980, while a laboratory scale system for liquefaction was disclosed at the above-mentioned Hartford Conference in a paper by P. M. Molton entitled, “Batelle Northwest Sewage to Fuel Oil Conversion”, consisting of alkaline pretreatment of the sludge and subsequent autoclaving at 320° C. for one hour at about 10,000 KPa under an argon atmosphere. This last process produces oil, asphalt and char with reported oil yields of up to 15% by weight of total sludge solids, total thermal efficiency of up to 70%, and net energy production of about 5.9 MJ/kg, the latter figure being based on the assumption that the oil represents the net energy.
In another process described by W. L. Kranich, K. Guruz, and A. H. Weiss in a paper entitled, “Hydro-liquefaction of Sewage Sludge”, published in the Proceedings of the National Conference on Municipal and Industrial Sludge Utilization and Disposal”, Washington, D.C., U.S.A., May 1980, both raw and digested dry sludge were processed with a carrier oil in an autoclave at temperatures ranging from 396° C.-420° C. under hydrogen at 10,000-13,000 KPa. Oils and asphaltenes were produced, with oil yields of up to 30%.
A process for the conversion of sewage sludge to produce oils has been disclosed in European Patent Application No. 81109604.9 filed Nov. 10, 1981 by Prof. Dr. Ernst Bayer and published May 26, 1982 (Pub. No. AZ 0 052 334), and has been described by E. Bayer and M. Kutubuddin of Tubingen University, Federal Republic of Germany, in several articles, for example in “Oil aus Mull and Schlamm” on pages 68-77 of Bild der Wissenschaft, Issue 9 (1981); in “Oil aus Klarschlamm” on pages 377-381 of Abwasser, Issue 29 (1982); and in “Low Temperature Conversion of Sludge and Water to Oil” in the Proceedings of the International Recycling Congress, 1982, Berlin, Federal Republic of Germany. The process has been demonstrated on both batch and continuous laboratory scale systems, and basically comprises heating dried sludge slowly with the exclusion of air to a conversion temperature of 280° C.-600° C. for about 30-180 minutes, the vapors being condensed to generate a crude oil and the solid residue being coal-like. Significant advantages of the process are stated to be that it can be operated at only slightly above atmospheric pressure and no additives are required, the developers postulating that catalyzed vapor phase reactions occur in which the organic materials are converted to straight chain hydrocarbons, much like those present in crude oil. Analysis of the product is stated to confirm that aliphatic hydrocarbons are present in contrast to other known oil-producing processes, which appear to tend to produce aromatic and cyclic compounds, whether utilizing sludge, cellulose or refuse as the substrate. The developers state that they have demonstrated oil yields ranging from 18-27% and char yields from 50-60% the oil having a heating value of about 39 MJ/kg and the char of about 15 MJ/kg. Energy balance calculations indicate that the process is a net producer of energy, provided that the sludge is first mechanically dewatered to about 20% solids, and it has been estimated that a net energy production of 10-15 MJ/kg solids could be obtained in a full scale process.
This Bayer process is simple and in effect, mimics the natural process of oil synthesis. It is known that natural crude oil was formed from microscopic organisms over geologic periods of time, and comprises a mixture of saturated and unsaturated hydrocarbons including some non-hydrocarbon material. It is postulated by Bayer that at the levels of energy input used, with the exclusion of oxygen, the proteins and lipids in the sludge are converted to oil and the carbohydrates to coal-like material, the process being catalyzed if necessary by the addition of suitable materials. It is stated that in the case of sewage sludge it is in most cases superfluous to add a catalyst material, since the inorganic components present in the sludge contain a sufficient amount of catalyst in the form of silicates, aluminum compounds and transition metals. The hetero-bonds (C—S, C—N, C—P, C—O) are broken, but not the C—C bonds, resulting in a hydrocarbon mix very similar to natural crude oil. The research indicated that the maximum oil yield was achieved at an operating temperature of 280° C. to 320° C.
In a solid waste treatment process disclosed in U.S. Pat. No. 3,714,038, issued Jan. 30, 1973 to the Black Clawson Company, a slurry is formed of a mixture of the organic and inorganic materials are then removed. The slurry is dewatered and pyrolized or hydrogenated to result in a series of products such as gas, oil, char and residue.
U.S. Pat. No. 3,962,044, issued Jun. 8, 1976 to the Regents of the University of California, proposes a process for the treatment of solid animal and human excreta by particulating and heating it in a closed heating zone at 200-1000° C. (300-600° C. preferred) for a period of 15-120 minutes, when a part is volatilized and the solid residue is carbonized. The volatilized portion is removed to a recovery zone and condensable are condensed there from, it being separated into aqueous, non-aqueous and non-condensable fractions.
U.S. Pat. No. 4,030,981, issued Jun. 21, 1977 to H. V. Hess, W. F. Frang and E. L. Cole describes processes for making low sulfur oil by coking wastes, one of which is sewage sludge, at temperatures of 400-550° F. pressures of 300-3000 p.s.i.g., and times of 5 minutes to 2 hours and thereafter reacting the coked waste with hot pressurized synthesis gas (carbon monoxide and hydrogen), the synthesis gas reaction employing temperatures of 400° F.-550° F. pressures of 300-3000 p.s.i.g., and times of 5 minutes to 2 hours and thereafter reacting the coked waste with hot pressurized synthesis gas (carbon monoxide and hydrogen), the synthesis gas reaction employing temperatures of 500° F.-750° F. and pressures of 500-5000 p.s.i.g.
U.S. Pat. No. 4,098,649, issued Jul. 4, 1978 to Redken-Young Processes Inc. describes a process for destructive distillation of organic material separated, for example by flotation, from industrial and municipal wastes in which the material is delivered to a screw extruder conveyor which is heated to different temperatures in succeeding zones along its length, for example 40° F.-600° F. in a first zone and up to 1500° F. in subsequent zones, the resultant char being discharged. The gaseous products are removed separately from the different zones and separated, and may include olefins and paraffin's.
U.S. Pat. No. 4,210,491 issued Jul. 1, 1980 to Tosco Corporation also proposes the use of a screw conveyor as a retort for converting substances containing organic material into hydrocarbon vapors and solid residue, the volatile materials being removed at different points along its length and subsequently processed. The retort conveyor is heated by a fluidized bed.
U.S. Pat. No. 4,344,770 issued Aug. 17, 1982 to Wilwardco Inc. discloses a process and apparatus intended principally for the hydrolysis treatment of sawdust and wood chips, but applicable also to sewage sludge. The separated gases are condensed to liquid and gas phases and the liquid phase is then separated by gravity into water and oil fractions. The water fraction is distilled to separate water soluble oils and they are added to the oil fraction to increase its energy content.
Canadian Patent No 1,075,003 issued Apr. 8, 1980 to Karl Klener describes a process for the production of combustible gas from waste materials, including sewage sludge, requiring drying of the material, its carbonization at low temperature (300-600° C.) in a first series of rotary tubes, separation of the resultant combustion components and conversion of the low temperature carbonization gases in a reaction bed of solid carbon at high temperature (1000-1200° C.).
Canadian Patent No 1,100,817 issued May 12, 1981 to Ahlstrom (A) Osakeyhtiz discloses a method of treating material, such as sewage sludge, in a fluidized bed reactor for its incineration, the process employing mechanical dewatering to achieve a high enough solids content for the process to be autogenous and not to require supply of auxiliary fuel. It is not always possible to remove sufficient water mechanically and the thus-dried material is fed first into a pre-reactor into which is passed hot separated solids removed from the main fluidized bed reactor, these hot solids being mixed thoroughly with the sludge in the pre-reactor to heat and dry it before it passes to the main reactor.
Canadian Patent No. 1,001,493, issued Dec. 14, 1976 to Phillips Petroleum Company, USA discloses a two-stage incinerator for waste products, such as sewage sludges. In the first stage vaporization or volatilization is achieved with some combustion occurring, and then all the gaseous products are conducted to a second stage in which further oxidation and combustion occurs, the hot flue gases from the second stage being quenched with cool air to provide preheated air for the combustion in either or both of the two stages.
U.S. Pat. No. 4,618,735 issued Oct. 21, 1986 to T. R. Bridle and H. W. Campbell disclosed a new process for the conversion of the organic components of sludge, particularly sewage, to produce useful, storable, energy-containing oil products, apparatus for carrying out the process and a control process for optimization of the process temperature. The sludge preferably is mechanically dewatered to about 20-25% solids by weight and thermally dried to about 90% solids by weight. The resultant material is comminuted and heated in the apparatus of the invention to at least 250° C. in a heating zone in the absence of oxygen to generate a gaseous atmosphere containing volatiles; this atmosphere is then removed, scrubbed of H2S and NH3 if required, and passed preferably in countercurrent flow in a intimate contact with the “devolatilized” sludge solids from the heating zone, again in the absence of oxygen, at a temperature of at least 280° C., resulting in catalyzed vapor phase oil-producing reactions. The oil vapors are carried out by the gas flow and condensed.
The apparatus moves the sludge solids co-current with the heating zone atmosphere and countercurrent with the reaction zone atmosphere. In the reaction zone the conveyor not only moves the comminuted sludge along but lifts it and drops it through the gaseous atmosphere of the preferred intimate contact. Preferably the sludge is examined repeatedly by differential scanning calorimetry to generate a thermogram which has been found to reveal immediately the optimum temperature for of the reaction zone, and also the anticipated oil yield from the sludge.
A practical problem with many of the proposed and employed hitherto, particularly those involving pyrolysis and incineration, is that the principal usable energy-containing products are gases, often not easily condensable, and of low net energy content, so that they are impossible or uneconomic to store and must be used immediately. Generally it is only practicable to use them to produce relatively low grade energy, such as steam, and flare them to waste during periods of little or no demand. There is a growing demand for processes that result in storable (liquid or liquefiable), transportable and if possible upgradeable energy or chemicals containing products, such as synthetic oils, with efforts directed to the optimum production of net storable energy of fine chemicals and with the non-storable products, if used at all, used in the operation of the process.
U.S. Pat. No. 7,276,148 by the present inventor, incorporated herein by reference, describes a combined thermal extraction/pyrolysis multi stage process for the conversion of the sludge, particular sewage sludge by heating and chemically reacting, in order to obtain a useful storable product therefrom, such as oils, in yields greater than obtained only in pyrolysis processes. It is known that during solid fuel pyrolysis, both the liquid and the gaseous products of the process leave the reactor in the vapor phase and are thus separated from the solid residue. This advantage was used of pyrolysis to separate the liquid products that are formed at the stage of thermal extraction of the feed stock.
The main disadvantage of said process is that the part of the liquid products, especially of the heavy fractions, which were created at the extraction stage, are transformed into coke at the pyrolysis stage at temperatures>400° C. This causes to loss of the part of liquid products, which were transferred into coke.
A comparative Table 3 below illustrates the advantages and disadvantages of the two methods of sewage sludge liquefaction—thermal extraction and pyrolysis.
As can be seen from Table 3 data, pyrolysis process convert part of liquid products (basically heavy fractions) to coke, which leads to yield decreasing.
TABLE 3Advantages and Disadvantages of Liquefaction Methods.LiquefactionmethodAdvantagesDisadvantagesThermal1. High yield of liquid products.1. The need to separate liquid productsextraction2. Absence of pyrolytic water infrom solid residue.in H-donor solventproducts.2. Lower calorific value of products.medium3. Higher conversion of sewagesludge O.M. into liquefiedproducts.Pyrolysis1. Higher calorific value of1. Low yield of liquid products.products.2. Conversion of part of organic matter2. Separation of liquid productsinto pyrolytic water.in vapor phase.3. Conversion a part of liquid productsinto coke.